An implantable cardioverter-defibrillator (ICD) is a member of a class of devices called implantable medical devices (IMD). Conventional pacemakers and ICDs as well as other IMDs use electronic sense amplifiers to detect the electrical signals arising from spontaneous or inherent electrical activation in the heart. Modern single chamber ICDs have a dedicated lead system, usually but not always transvenous, coupled to the right ventricle of the heart. The typical lead system will have a defibrillation coil electrode inside the right ventricle, and either another coil, usually in the superior vena cava (SVC) and/or the generator case serves as the other pole in the shocking circuit. The typical lead will also have a bipolar pair of electrodes at the tip of the lead system near the endocardium of the right heart. This pair of electrodes includes a distal tip electrode and a more proximal ring electrode or distal coil. It is this pair of electrodes that is usually connected to a sense amplifier in the ICD. The electromyographic waveform arises in part from the passage of a depolarization wavefront event or R-wave, past the electrode site. The band pass and gain characteristics of the sense amplifier is “tuned” to discriminate this relatively large (voltage) physiologic signal (EMG) from myopotentials and other electrical noise signals that also may be picked up by the electrode or intercepted by the implanted lead system.
A significant problem occurs when there is enough energy presented to the sense amplifier to “trick” the sense amplifier into mistaking noise for a cardiac signal. External sources of noise include both ambient electrical noise such as electromagnetic interference (EMI) as well as artifact from compromises in lead integrity. This external noise or artifact can lead to significant clinical problems. Since the relative timing of one detected cardiac event to the next detected cardiac event is used for rhythm analysis and the invocation of therapy, such noise can inhibit the proper operation of the IMD. For example, in the time domain, two or more R-waves may occur close together in time indicating that a tachycardia or fibrillation may exist. The device may be programmed to intervene and aggressively treat the heart if several rapid beats are detected in a row. In this example, the detection of noise and its incorrect interpretation as a sequence of several fast R-waves may result in treatment where none is required. Or the detection of noise may mask the detection of the absence of normal underlying ventricular rhythm and therefore pacing therapy will be withheld when in fact it is required. Both false positives and false negatives may be harmful.
Another significant problem arises from the inability of the ICD to distinguish ventricular tachycardia—a heart rhythm that originates from the ventricles—from supraventricular tachycardia such as atrial fibrillation—a heart rhythm that originates from the atria. Rapidly conducted signal from the atria to the ventricles may be interpreted inappropriately as ventricular tachycardia and so inappropriate shock therapy may be delivered. In this instance, both supraventricular tachycardia and ventricular tachycardia may have rates that exceed a time domain rate cutoff value.
It is recognized that efforts need to be devoted to improving the ability of implanted devices to detect and correctly interpret heart rhythms.